Photochemistry and environment

The transformation mechanisms of the three monochlorophenols in dilute aqueous solution are studied through the comparison of direct excitation and the use of photoexcited phenol. The primary photochemical process is a C‐Cl bond scission which is not influenced by oxygen. However, the position of the chlorine on the ring strongly influences the nature of this scission. The direct irradiation of 3‐chlorophenol yields to resorcinol whatever the pH, and the reaction can be photosensitized by phenol. A mechanism of direct photohydrolysis from the triplet state is proposed. Under conditions of direct excitation, 2‐chlorophenol can be deprotonated leading to a cyclopentadienic acid or hydrolysed to pyrocatechol. In the presence of excited phenol, radical mechanisms produce biphenyl derivatives. With 4‐chlorophenol, photohydrolysis appears to be a minor process, whereas the major pathway is a radical mechanism which can be induced by phenol excitation.